The rapid increase in the use of fossil fuels has accelerated the demand for alternative energy sources or clean energy sources, and research has been actively carried out into power generation and power storage using electrochemistry.
A typical example of an electrochemical device using such electrochemical energy is a secondary battery, which has been increasingly used in various fields.
Based on the shape of a battery case, secondary batteries may be classified into a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, and a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case made of an aluminum laminate sheet. The cylindrical battery has advantages in that the cylindrical battery has a large capacity and in that the cylindrical battery is structurally stable.
In addition, the electrode assembly mounted in the battery case functions as a power generating element, having a positive electrode/separator/negative electrode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly-roll type electrode assembly configured to have a structure in which a long sheet type positive electrode and a long sheet type negative electrode, to which active materials are applied, are wound in the state in which a separator is disposed between the positive electrode and the negative electrode or a stacked type electrode assembly configured to have a structure in which a plurality of positive electrodes having a predetermined size and a plurality of negative electrodes having a predetermined size are sequentially stacked in the state in which separators are disposed respectively between the positive electrodes and the negative electrodes. The jelly-roll type electrode assembly has advantages in that it is easy to manufacture the jelly-roll type electrode assembly and in that the jelly-roll type electrode assembly has high energy density per unit weight.
Meanwhile, lithium-containing cobalt oxides, such as LiCoO2, are mainly used as positive electrode active materials for lithium secondary batteries. In addition, lithium-containing manganese oxides, such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxides, such as LiNiO2, are also used.
Among positive electrode active materials, LiCoO2 is widely used due to its excellent overall physical properties, such as excellent cycle properties. However, LiCoO2 is low in safety and expensive due to limited resources of cobalt, which is a raw material therefor. Lithium nickel-based oxides, such as LiNiO2, are cheaper than LiCoO2, and exhibit a high discharge capacity when charged to a voltage of 4.25 V. However, lithium nickel-based oxides have problems, such as high production cost, swelling due to gas generated in batteries, low chemical stability, and high pH.
In addition, lithium manganese oxides, such as LiMnO2 and LiMn2O4, are advantageous in that they contain manganese, which is an abundant and environmentally friendly raw material, and thus are drawing much attention as a positive electrode active material that can replace LiCoO2. In particular, among the lithium manganese oxides, LiMn2O4 has advantages, such as a relatively inexpensive price and high output. On the other hand, LiMn2O4 has lower energy density than LiCoO2 and three component-based active materials.
In order to overcome these disadvantages, some of the Mn in LiMn2O4 is substituted with Ni and thereby LiMn2O4 has a higher potential (approximately 4.7 V) than its original operating potential (approximately 4 V). Due to the high potential, a spinel material having a composition of Li1+aNixMn2−xO4−z (0≤a≤0.1, 0.4≤x≤0.5, and 0≤z≤0.1) is well suited to use as a positive electrode active material of a middle or large-sized lithium ion battery for electric vehicles (EV), which require high energy and high-output performance. However, the lifespan characteristics of the battery may be reduced due to dissolution of Mn in the positive electrode active material and side reactions of an electrolytic solution caused by the high charge and discharge voltage potential.
Therefore, there is a high necessity for technology that is capable of improving the lifespan characteristics of a battery while using a positive electrode active material containing a high content of Mn, as described above.